1. Technical Field
The present invention relates to a drive control method of an electrostatic-type ultrasonic transducer which generates constant high sound pressure in a wide frequency band range, the electrostatic-type ultrasonic transducer, an ultrasonic speaker using the electrostatic-type ultrasonic transducer, an audio signal reproducing method, a superdirectional acoustic system, and a display.
Priorities of Japanese Patent Application No. 2005-353275 filed on Dec. 7, 2005 and Japanese Patent Application No. 2006-307860 filed on Nov. 14, 2006 are claimed, and the entire disclosures of these are incorporated by reference herein.
2. Background Art
Currently, most of ultrasonic transducers are of resonance-type using piezoelectric ceramics.
A structure of a related-art ultrasonic transducer is shown in FIG. 15. Most of ultrasonic transducers currently used are of resonance-type using piezoelectric ceramics as oscillation elements. The ultrasonic transducer shown in FIG. 15 converts electric signals into ultrasonic waves and converts ultrasonic waves into electric signals (transmission and reception of ultrasonic waves) using piezoelectric ceramics as oscillation elements. A bimorph-type ultrasonic transducer shown in FIG. 15 has two piezoelectric ceramics 61 and 62, a cone 63, a case 64, leads 65, 66, and a screen 67.
The piezoelectric ceramics 61 and 62 are affixed to each other, and the lead 65 is connected with one of the surfaces opposite to the affixed surfaces of the piezoelectric ceramics 61 and 62, and the lead 66 is connected with the other surface.
Since the resonance-type ultrasonic transducer uses resonance phenomena of the piezoelectric ceramics, the characteristics of ultrasonic waves in transmission and reception are excellent in a relatively narrow frequency band range around the resonance frequency.
Different from the resonance-type ultrasonic transducer shown in FIG. 15, an electrostatic-type ultrasonic transducer is known as a wideband-type ultrasonic transducer which can generate high sound pressure over a high frequency band range. The electrostatic-type ultrasonic transducer is called a pull-type transducer since its oscillation film acts only in a direction to be attracted toward a fixed electrode. A specific structure of the wideband-type ultrasonic transducer (pull-type) is shown in FIG. 16. The electrostatic-type ultrasonic transducer shown in FIG. 16 uses a dielectric 131 (insulator) such as PET (polyethylene terephthalate resin) having a thickness of about 3 to 10 μm as an oscillator. An upper electrode 132 as a metal leaf made of aluminum or other materials is formed on the upper surface of the dielectric 131 as one piece by evaporation or other methods, and a lower electrode 133 made of brass is formed on the lower surface of the dielectric 131 in contact with each other. The lower electrode 133, with which a lead 152 is connected, is fixed to a base plate 135 made of bakelite or other materials.
A lead 153 is connected with the upper electrode 132 and a DC bias power supply 150. The DC bias power supply 150 constantly applies DC bias voltage of about 50 to 150 V to the upper electrode 132 to attract the upper electrode 132 toward the lower electrode 133. A signal supply 151 is equipped.
The dielectric 131, the upper electrode 132 and the base plate 135 are caulked by a case 130 with metal rings 136, 137 and 138, and a mesh 139.
A plurality of small grooves having non-uniform shapes and sizes of several tens to hundreds micrometers are formed on the surface of the lower electrode 133 facing the dielectric 131. Since the small grooves produce clearances between the lower electrode 133 and the dielectric 131, capacitance distribution between the upper electrode 132 and the lower electrode 133 varies with small fluctuations. These random small grooves are formed by roughing the surface of the lower electrode 133 by hand using a file. Since a number of capacitances with clearances having different sizes and depths are formed on the electrostatic system ultrasonic transducer, the ultrasonic transducer shown in FIG. 16 exhibits wideband frequency characteristics as indicated by a curve Q1 shown in FIG. 17.
According to the ultrasonic transducer having this structure, rectangular-wave signals (50 to 150 V p-p) are applied between the upper electrode 132 and the lower electrode 133 while DC bias voltage being applied to the upper electrode 132. The resonance-type ultrasonic transducer has frequency characteristics indicated by a curve Q2 in FIG. 17 having a center frequency (resonance frequency of piezoelectric ceramic) of 40 kHz, for example. Thus, the maximum sound pressure minus 30 dB is generated in the range of ±5 kHz from the center frequency where the maximum sound pressure is generated.
On the other hand, the frequency characteristics of the wideband-type ultrasonic transducer having the above structure are flat from about 40 kHz to about 100 kHz, and ±6 dB from the maximum sound pressure is generated at 100 kHz (see Patent References 1 and 2).    [Patent Reference 1] JP-A-2000-50387    [Patent Reference 2] JP-A-2000-50392
As discussed above, the electrostatic system ultrasonic transducer shown in FIG. 16 is known as a wideband ultrasonic transducer (pull type) which can generate relatively high sound pressure in a wide frequency band, different from the resonance-type ultrasonic transducer shown in FIG. 15. As shown in FIG. 13, the maximum sound pressure of the resonance-type ultrasonic transducer is 130 dB or larger. However, the maximum sound pressure of the electrostatic-type ultrasonic transducer generates sound pressure of only 120 dB or lower, which is slightly insufficient when the transducer is used for an ultrasonic speaker.
The details of an ultrasonic speaker are herein explained. Amplitudes of signals in an ultrasonic frequency band range called carrier waves are modulated by audio signals (signals in audio frequency band), and the ultrasonic transducer is operated based on the modulation signals. Then, sound waves produced from ultrasonic waves modulated by the audio signals of the signal supply are released in the air, and the original audio signals are self-reproduced in the air by non-linearity of the air.
Since sound waves are condensational and rarefactional waves which transmit in the air as transmission medium, the difference between the condensational part and the rarefactional part of the air becomes prominent during transmission of the modulated ultrasonic waves. That is, the speed of sound is high in the condensational part, and the speed of sound is low in the rarefactional part. Thus, distortion of the modulated waves is caused, resulting in waveform separation of the modulated waves into carrier waves (ultrasonic waves) and audio waves (original audio signals). In this case, humans can hear only audio sounds (original audio signals) at frequencies lower than 20 kHz. This principle is generally called parametric array effect.
For utilizing sufficient parametric array effect, the ultrasonic wave sound pressure needs to be at least 120 dB. However, it is difficult for the electrostatic-type ultrasonic transducer to achieve this level, and thus a ceramic piezoelectric device such as PZT or a polymeric piezoelectric device such as PVDF is often used as an ultrasonic wave generator.
However, a piezoelectric device has a sharp resonance point regardless of its material, and is actuated at the corresponding resonance frequency for practical use as an ultrasonic speaker. Thus, the frequency range where high sound pressure is securely generated is extremely narrow. That is, the piezoelectric device has a narrow band.
Generally, the maximum audio frequency band for humans is considered in the range from 20 Hz to 20 kHz, and thus humans have approximately 20 kHz band range. It is therefore possible to accurately demodulate original audio signals only when high sound pressure is secured over the frequency band range of 20 kHz in the ultrasonic wave range. It is easily understood that accurate reproduction (demodulation) in the wide range of 20 kHz is absolutely impossible when the conventional resonance-type ultrasonic speaker having the piezoelectric device is used.
Actually, the ultrasonic speaker using the conventional resonance-type ultrasonic transducer has the following problems: (1) narrow band and poor reproduction sound quality; (2) the allowable modulation factor is only about 0.5 or lower since demodulated sounds are distorted at an excessively high AM factor; (3) oscillation of the piezoelectric device becomes unstable and sounds are split when input voltage (volume) is increased, and the piezoelectric device itself tends to be broken when voltage is further increased; and (4) arraying, size-increasing and size-reducing are difficult, which leads to higher cost. The ultrasonic speaker using the electrostatic-type ultrasonic transducer (pull type) shown in FIG. 16 can solve almost all the problems arising from the above related art. However, absolute sound pressure required for sufficient sound volumes of demodulated sounds is short even though the band is widely covered.
Additionally, according to the pull-type ultrasonic transducer, electrostatic force acts only in the direction of attraction toward a fixed electrode, and the oscillation symmetry of an oscillation film (corresponding to upper electrode 132 in FIG. 16) is not maintained. Thus, in case that the pull-type transducer is used in the ultrasonic speaker, there is a problem that oscillations from the oscillation film directly generate audio sounds.
In order to overcome these drawbacks, the inventors of the invention have already proposed an ultrasonic transducer which can generate acoustic signals at a sufficiently high sound pressure level for obtaining parametric array effect in a wide frequency band range. According to this ultrasonic transducer, an oscillation film having a conductive layer is sandwiched by a pair of fixed electrodes having through holes at opposed positions, and AC signals are applied to a pair of the fixed electrodes while DC bias voltage being applied to the oscillation film.
This ultrasonic transducer is called push-pull-type ultrasonic transducer. According to this transducer, the oscillation film sandwiched between a pair of the fixed electrodes simultaneously receives electrostatic attraction force and electrostatic repulsive force in the same direction in accordance with the polarity direction of the AC signals. Thus, the oscillations of the oscillation film can be increased to a sufficient level for obtaining the parametric array effect. Moreover, since the oscillation symmetry is secure, higher sound pressure than that of the conventional pull-type ultrasonic transducer can be generated over a wide frequency band range.
However, it is difficult for the push-pull-type ultrasonic transducer to generate sufficient sound pressure in the air since the through holes through which sounds are released have relatively small areas.
Therefore, improved techniques for generating sufficient sound pressure are also required for the push-pull-type ultrasonic transducer having the above structure.
In addition, additional values of the ultrasonic transducer can be offered if the ultrasonic transducer generates high sound pressure over a wide band range.